The present invention relates to a rotary tube, especially for a rotary tube kiln for the production of activated carbon, in accordance with the disclosure as well as to a rotary tube kiln with such a rotary tube. Furthermore, the present invention relates to the use of this rotary tube and/or rotary tube kiln for the production of activated carbon and to a method for the production of activated carbon using this rotary tube and/or rotary tube kiln.
Activated carbon is the most-used adsorbent on account of its quite non-specific adsorbent properties. Legal requirements as well as the rising consciousness of a responsibility for the environment are resulting in an increasing need for activated carbon.
Activated carbon is being increasingly used in the civilian and in the military area. In the civilian area activated carbon is used, e.g., for the cleaning of gases, filter systems for air conditioning, automobile filters, etc. whereas in the military area activated carbon is used in all types of protective materials (e.g., gas masks, protective coverings and protective garments of all types such as, e.g., protective suits, etc.).
Activated carbon is generally obtained by carbonization (also designated synonymously as low-temperature carbonization, pyrolysis or coking) and by subsequent activation of suitable carbon-containing raw materials. Those raw materials are preferred that result in economically reasonable yields because the weight losses due to the splitting off of volatile components in the carbonization and to the roasting residue during activation are considerable. For further details one can refer to, e.g., H. v. Kienle and E. Bäder, Aktivkohle und ihre industrielle Anwendung [Activated Carbon and Its Industrial Use], Enke Verlag Stuttgart, 1980.
The nature of the activated carbon produced, fine-pored, or coarse-pored, solid or brittle, is a function of the carbon-containing raw material. Customary raw materials are, e.g., coconut shells, wood chips, peat, hard coal, tars, and in particular plastics such as, e.g., sulfonated polymers, that play a large part, among other things, in the production of activated carbon in the form of granules or spherules.
Activated carbon is used in various forms: powdered carbon, splint carbon, granular carbon, molded carbon and also, since the end of the '70s, granular and spherical activated carbon (so-called “granular carbon” or “spherical carbon”). Granular carbon, especially spherical activated carbon, has a number of advantages over other forms of activated carbon such as powdered carbon, splint carbon and the like, that make it valuable or even absolutely necessary for certain applications. It is flowable, enormously wear-resistant and dust-free and very hard. Granular carbon, especially spherical carbon, is very much in demand on account of its special form and also on account of the extremely high wear-resistance for special areas of use, e.g., surface filter materials for suits for protection against chemical poisons or filters for low concentrations of noxious substances in large amounts of air.
In most instances suitable polymers are used as starting material in the production of activated carbon, especially granular carbon and spherical carbon. Sulfonated polymers, especially sulfonated divinylbenzene-crosslinked styrene polymers are preferably used, in which case the sulfonation can also be achieved in situ in the presence of sulfuric acid or oleum. Suitable raw materials are, e.g., ion-exchange resins or their precursors, which are usually divinylbenzene-crosslinked polystyrene resins. In the case of finished ion exchangers, the sulfonic acid groups are already present in the material and in the case of the ion exchanger precursors, they must still be introduced by sulfonation. The sulfonic acid groups have a decisive function since they play the part of a crosslinking agent in that they are split off in the carbonization. However, a particular disadvantage and problem is constituted by the large amount of released sulfur dioxide, as well as associated corrosion problems, among other things, in the production equipment.
Activated carbon is usually produced in rotary tube kilns. These have, e.g., a charging area for the providing of raw material at the beginning of the kiln and a discharge area for the end product at the end of the kiln.
In the traditional processes for the production of activated carbon in accordance with the state of the art in discontinuous production, the carbonization and the subsequent activation take place in a single rotary tube.
During the carbonization, which can be preceded by a phase of pre-carbonization or pre-low-temperature carbonization, the conversion of the carbon-containing raw material to carbon takes place; that is, in other words, the raw material is carbonized. During the carbonization of the previously cited organic polymers based on styrene and divinylbenzene that contain crosslinking functional chemical groups, which result during their thermal decomposition in free radicals and therefore in crosslinkings, especially sulfonic acid groups, the functional chemical groups, in particular sulfonic acid groups, are destroyed under the splitting-off of volatile components such as, in particular, SO2, and free radicals are formed that bring about a strong crosslinking without which there would be no pyrolysis residue (=carbon). Suitable initial polymers of the previously-cited type are, in particular, ion-exchange resins (e.g., cation-exchange resins or acidic ion-exchange resins, preferably with sulfonic acid groups, e.g., cation-exchange resins based on sulfonated styrene/divinylbenzene copolymers) or their precursors (that is, non-sulfonated ion-exchange resins that must still be sulfonated before or during the carbonization with a suitable sulfonation agent such as, e.g., sulfuric acid and/or oleum). In general, the pyrolysis is carried out under an inert atmosphere (e.g., nitrogen) or possibly under a slightly oxidizing atmosphere. It can likewise be advantageous to add a rather small amount of oxygen, especially in the form of air (e.g., 1 to 5%), to the inert atmosphere during the carbonization, especially at rather high temperatures (e.g., in a range of approximately 500° C. to 650° C.) in order to bring about oxidation of the carbonized polymer skeleton and to facilitate the subsequent activation in this manner.
The carbonization is then followed by the activation of the carbonized raw material. The basic principle of the activation consists in degrading selectively and purposefully under suitable conditions a part of the carbon generated during the low-temperature carbonization. This produces numerous pores, cracks and fissures and the surface of the activated carbon increases considerably relative to the unit of mass. Thus, a purposeful calcining of the carbon is carried out during the activation. Since carbon is degraded during the activation, a partial, considerable substance loss occurs during the procedure and is equal to an increase in the porosity under optimal conditions and signifies an increase in the inner surface (pore volume) of the activated carbon. Therefore, the activation takes place under selective conditions that oxidize in a controlled manner. Customary activation gases are in general oxygen, especially in the form of air, water vapor and/or carbon dioxide as well as mixtures of these activation gases. Inert gases (e.g., nitrogen) can be added, if necessary, to the activation gases. In order to achieve a technically sufficiently high reaction rate, the activation is carried out in general at relatively high temperature, especially in a temperature range of 700° C. to 1,200° C., preferably 800° C. to 1,100° C. This places high requirements on the temperature resistance of the rotary tube material.
It is in addition necessary in the framework of the production of activated carbon in rotary tube kilns that a thorough mixing of the carbon-containing or carbonized raw material take place in the rotary tube with the carbonization and the activation, because a thorough mixing of the raw material ensures that, e.g., the acidic reaction products split off during the carbonization can be uniformly and effectively removed from the carbon-containing raw material. A thorough mixing of the carbonized raw material is also desirable in the method step of activation since, given the background of the production of a homogeneous active carbon material, a uniform contact of the carbonized raw material with the activation gases is desirable. Therefore, a thorough mixing of the raw material in the framework of the production of activated carbon in rotary tube kilns can make a significant contribution to the obtention of a homogeneous and efficient activated carbon product.
However, a thorough mixing of the raw material as is ensured in particular by a thorough axial mixing, that is, a thorough mixing or mixing along the longitudinal axis of the rotary tube, is not always ensured to a sufficient extent by traditional rotary tubes of the state of the art. In particular, such rotary tubes of the state of the art do not have a good thorough axial mixing of the raw material. This serious disadvantage of the rotary tubes of the state of the art can also be traced back to the cause that such rotary tubes have a constant inside diameter and that the rotary tubes therefore are designed solely in a cylindrical form. The mixing elements that are occasionally provided in the rotary tubes of the state of the art and arranged in the inside space of the rotary tubes cannot make a significant contribution toward ensuring an intense axial mixing of the raw material or charged material in this connection, in particular on account of their limited dimensions and their arrangement inside the cylindrical tube. A concrete coordination between the rotary tube geometry on the one hand and arrangement and formation of the mixing elements on the other hand, given the background of improving the axial mixing of the raw material for obtaining a more homogeneous end product, is not provided in the state of the art. Thus, rotary tubes of the state of the art have the disadvantage of an thorough axial mixing of the raw material or charged material that is not always optimal, so that as a consequence the resulting end product is not always optimized as regards its homogeneity.
Therefore, given this technological background, the present invention has the problem of making an apparatus or a rotary tube available that is suitable in particular for the production of activated carbon and with which the previously described disadvantages of the state of the art are at least partially avoided or at least are attenuated. In particular, a rotary tube should be made available that results in an improved thorough axial mixing of the raw material or of the charged material so that especially homogeneous activated carbon materials can be produced as end products with a uniformly great pore volume with one of such rotary tubes.
In order to solve the previously described problem, the present invention suggests a rotary tube in accordance with the disclosure. Further advantageous embodiments constitute subject matter of the dependent claims.
Further subject matter of the present invention is constituted by a rotary tube kiln according to the disclosure that comprises the rotary tube in accordance with the invention.
Finally, further subject matter of the present invention is constituted by the use of the rotary tube or rotary tube kiln in accordance with the invention to produce activated carbon in accordance with the disclosure, that is, more precisely, a method for the production of activated carbon by carbonization and the subsequent activation of carbon-containing raw materials, which method is carried out in a rotary tube or rotary tube kiln in accordance with the present invention.
Therefore, subject matter of the present invention, in accordance with a first aspect of the present invention, is a rotary tube, in particular for a rotary tube kiln for the production of activated carbon with several mixing elements arranged in the inner space of the rotary tube for thoroughly mixing a charged material, which rotary tube comprises a transitional area from a smaller inside cross section to a larger inside cross section of the rotary tube, in which mixing elements are formed or arranged in the transitional area in such a manner that the charged material is transported during operation by the mixing elements to the smaller inside cross section.
The applicant surprisingly found that in particular the thorough axial mixing or blending of the charged material or of the raw material for the production of activated carbon can be considerably improved if the rotary tube has a transitional area from a smaller inside cross section to a larger inside cross section as well as mixing elements that transport the charged material during the operation of the rotary tube to the smaller inside cross section. At the same time an optimal radial or horizontal thorough mixing of the charged material is of course also achieved.
In this manner a rotary tube is created that results in an excellent thorough axial mixing, that is, in an excellent thorough mixing of the charged material along the longitudinal axis of the rotary tube of the invention, during which it is ensured that during the thorough mixing no unmixing or dehomogenization, that is, no splitting up of the charged material into fractions of larger and smaller particles and no grinding of the charged material takes place, which results in better homogeneity of the charged material and therewith also of the activated carbon resulting from it.
Therefore, a basic principle of the present invention is the fact that due to the special positioning and design of the mixing elements in the transitional area, a transport process by the mixing elements results to a certain extent that is counter to the transport process in the direction of the slope of the rotary tube and that results from the different inside cross sections of the transitional area. In other words, the mixing elements, especially the turning baffles, transport the charged material to a certain extent in the direction counter to the slope of the transitional area, which results in a thorough axial mixing of the charged material. This effectively avoids in particular a separation of the charged material into large or small particles and ensures a uniform contacting of the charged material, e.g., with the oxidizing atmosphere in the framework of the activation step, as a result of which an especially homogenous end product results.
Furthermore, the rotary tube of the invention can be equipped in accordance with an especially preferred embodiment of the invention with reinforcement elements on the outside in order to heighten the mechanical stability, in particular the ability to resist deformations at high operating temperatures. According to a further preferred embodiment of the invention, it can be additionally provided that the mixing elements are fastened or welded to the outside of the rotary tube, and are preferably connected to the rotary tube exclusively by the outside fastening or welding in order to protect the welding connection from the corrosive conditions in the internal space of the rotary tube and from the high operating temperatures prevailing there.
Further advantages, properties, aspects, particularities and features of the present invention result from the following description of a preferred exemplary embodiment shown in the drawings.